1. Field of the Invention
The present invention relates to seismic exploration, and, more particularly, to apparatus for use in a distributed seismic exploration system employing fiber optics as the medium of communication between remote units in such system.
2. Description of the Prior Art
In seismic exploration, sound waves are commonly used to probe the earth's crust as a means of determining the type and location of subsurface formations. The earth's crust can be considered a transmission medium or filter whose characteristics are to be determined by passing sound waves through that medium. In the reflection seismic method, sound waves or impulses are generated at a transmission point at or near the earth's surface, and sound waves reflected from subsurface reflecting boundaries are received at one or more receiving points. The received waves are detected and recorded in a form which permits analysis. Skilled interpreters can discern from such an analysis the shape and depth of subsurface reflection boundaries and the likelihood of finding accumulations of minerals, such as oil and gas.
In seismic exploration, seismic detectors, e.g., geophones, are arranged in arrays or nests at spaced intervals on the earth's surface, and generate electrical signals responsive to the reflected portion of the sound waves. By well known techniques, those electrical signals may be conveyed via cables to a location where they are recorded.
In recent years, the seismic industry has begun to use distributed systems for data acquisition. A typical distributed system may, for example, comprise a number of remote units which are located at spaced intervals on the earth's surface. Such remote units receive inputs from a plurality, e.g., eight, geophone arrays. Typically, such remote units operate to filter, amplify, and digitize the analog signals which are received from the geophone arrays. The remote units are serially connected to each other and to a central controller unit via an appropriate communication medium. Upon receipt of appropriate command word signals from the central controller unit, a remote unit will transmit the digitized seismic data to the central controller for storage and further processing.
One type of communication medium which has been utilized in distributed seismic exploration systems is a fiber optic cable. Examples of such distributed seismic exploration systems which have used fiber optic cables are the MDS-14 and MDS-16 systems, which are available from Geosource Inc., the assignee of the present application. One advantage of utilizing a fiber optic transmission medium in remote seismic telemetry systems is its relatively light weight, as compared to other communication media.
Problems have, however, existed in fiber optic distributed systems with respect to fiber optic cable lengths. As indicated above, in a typical distributed system, the seismic sensors are distributed in a linear pattern along the earth's surface. Depending upon the topography and objectives of the seismic survey, such sensors arrays may vary in length of surface coverage from over 440 ft. to below 25 ft. In a typical distributed system with eight sensors arrays connected to a remote processing unit, the separation between remote units may be as great as 8.times.440 ft. or approximately 3520 ft., thus requiring a fiber optic cables of that length between adjacent remote units. As will be readily appreciated, some difficulty has arisen with respect to the management and handling of such lengths of fiber optic cable. This problem has been solved by the apparatus of the present invention.